The present invention relates to a method for preparing a screen printing stencil. More specifically, this invention relates to a method for preparing a screen printing stencil by electroforming.
Screen printing stencils have a wide variety of uses in the electronic substrate fabrication and electronic assembly industries. These include, but are not limited to, the printing of conductors, resistors, dielectrics, solder masks, and solder pastes. Screen printing stencils currently are fabricated by "chemical milling" also known as chemical etching.
U.S. Pat. No. 3,610,143 to Greenwood discloses a rotary screen printing stencil produced by etching techniques. A mandrel is provided with a layer of etchable material on an outer surface thereof. A copper layer then is electroformed onto the mandrel, followed by applying a nickel layer and another copper layer. A resist then is coated on the mandrel followed by UV exposure and etching.
U.S. Pat. No. 4,915,983 to Lake et al. discloses a multilayer circuit board fabrication process which utilizes separately designed and fabricated composite structures consisting of a patterned conductive metal foil supported by a photo-processible insulating film. The patterns on the metal foil are formed in a conventional manner, such as by applying a photoresist layer to an epoxy fiber glass substrate coated with a copper foil and patterning the photoresist layer by exposing it to ultraviolet light through a stencil-like film mask. Areas exposed on the photoresist are polymerized. The unpolymerized areas are removed by a chemical solution, leaving areas of copper, in the desired conductor pattern, underneath the protective barrier of the remaining polymerized photoresist. The exposed copper then is etched away and the remaining photoresist is chemically removed to expose the resulting conductor pattern.
As the size of electronic assemblies becomes smaller, chemical etching is incapable of holding the required tolerances and generating sufficiently small features. Thus, there is a need for a method of preparing screen printing stencils which are capable of holding required tolerances and generating sufficiently small features.